1. Field of the Invention
The invention described and claimed herein relates to high temperature forming of metal products particularly for aerospace and similar uses. More particularly it relates to mold structures for such forming.
2. Background Information
Certain metals, such as titanium alloys, exhibit superplasticity at high temperatures. Superplasticity is characterized by the ability of these metals to exhibit tensile elongation far in excess of what other metals can exhibit without exhibiting local necking. Superplastic forming (SPF) methods have primarily been used to form various planar, complex contoured, as well as cylindrical titanium alloy aerospace parts, such as engine intakes, nozzles, combustion chambers and cowlings.
A well-known superplastic metalforming method includes the following steps. First, two titanium sheets are rolled and welded to form two cylinders, a “forming cylinder” and a “slave cylinder,” of the same length but slightly different diameters. The forming and slave cylinders are placed concentrically, with the slave cylinder inside the forming cylinder. The upper ends of the forming and slave cylinders are then welded together, as are their lower ends. One or more gas fittings are welded in place along the upper or lowerweld beads. The resulting assembly, known as a preform assembly, thus has a tubular chamber bounded by the inner wall of the forming cylinder, the outer wall of the slave cylinder, and the upper and lower weld beads. The welds seal the chamber gas-tight but for the gas fittings. The preform assembly is then placed over a mandrel, which typically consists of a sturdy steel cylinder having an outside diameter slightly less than the inside diameter of the preform assembly. A multi-piece generally cylindrical die is placed around the preform assembly. The die consists of several sector-shaped segments to allow it to be removed following forming, as described below. One or more containment bands are then placed over the die. It is known that using multiple containment rings spaced from one another rather than a single longer, cylindrical containment band is advantageous because the spaced, less massive rings heat more quickly during the heating step and cools more quickly during the cool-down step of the process. The entire assembly is then placed in a vacuum furnace and heated to a temperature at which the titanium exhibits superplasticity. Inert gas, such as argon, is introduced under pressure into the gas fittings. The gas pressure presses the slave sheet firmly against the mandrel and the forming sheet firmly against the inner surface of the die. The inner surface of the die reflects the desired shape of the part to be formed. The forming sheet thus conforms to the shape of the inner surface of the die. The gas pressure is then relieved and the assembly cooled. When the assembly has cooled, the containment rings and die segments are removed. The upper and lower edges of the formed metal assembly are trimmed to separate the portion that includes the formed part from the remaining portion, which formerly defined the slave sheet, portions of the welds, the gas fittings, handling tabs, and so forth. The formed part may then be further trimmed and finished in any suitable manner.
Another very common SPF method has been used for forming parts that are more planar and less cylindrical. The method is similar to the simple stamping methods that have long been used to form sheet metal parts. A generally flat or planar die half having a generally concave surface that reflects the shape of the part to be formed is placed horizontally in a “hot box” (a frame having a heating element), with the concave surface of the die facing upwardly. A titanium sheet is placed on top of this lower die half. The hot box then heats the titanium sheet to a temperature at which it will exhibit superplasticity. The upper portion of the press clamps down on the sheet/die combination and is brought up to SPF temperature. Gas pressure is applied to the sheet, causing it to form into the die. After forming is complete, the press top is raised and the sheet is removed, followed by immediate insertion of a new sheet, and a repeat of the forming cycle.
It was recognized that it would be desirable to have an SPF method and apparatus that enabled generally planar parts as well as parts of a more cylindrical shape to be formed without requiring the expensive press apparatus as well as more economical tooling. Such a method and apparatus were developed and have been described and claimed in U.S. Pat. No. 5,823,034 issued in 1998.
That invention incorporates a die which includes two or more die segments, each of which is unitarily formed from a suitable non-metallic material. Each die segment has a unitarily formed connecting portion for interlocking it to another die segment. When interlocked in this manner, the interior chamber of the die defines the shape of the part to be formed.
The interlocking of unitarily formed die segments obviates the need for external containment rings. It has been discovered that certain non-metallic materials are generally sufficiently strong to withstand the SPF process without external reinforcement. Moreover, such materials are preferred because they can readily be machined or cast to provide the interior chamber of the die with the desired shape. Other materials having equivalent strength and resistance to thermal expansion and that may be machined, cast or otherwise readily shaped may also be suitable. The absence of massive containment rings, which undesirably act as heat sinks in prior die assemblies, allows the die to heat rapidly during the heating step of the SPF process and cool rapidly during the cooling step. Furthermore, the absence of heavy containment rings facilitates handling of the die apparatus.
In certain embodiments of that invention, the die may swing open and closed on hinges. In such a hinged embodiment, the portions of the die that swing relative to one another each preferably comprise a single die segment, but multiple die segments would also be suitable. The die segments may include tabs, as described above. One or more pins extending through bores at one end of the die may define the hinges. Similarly, one or more pins may be extended through the bores at an opposite end of the die to removably interlock the die segments after swinging the die closed. Thus, the tabs and pins may hingedly interlock the two die portions at one end, and removably interlock the two die portions at an opposite other end.
The die may have any suitable shape, although the shape of the die may reflect the shape of the part to be formed in it. For example, to form a generally planar part, such as a body panel, the die may be generally planar. Similarly, to form a more cylindrical part, such as an exhaust nozzle, the die may be generally cylindrical. Nevertheless, the part may be formed inside the die in any suitable orientation and thus does not dictate the shape of the die.
To use the die in the SPF method, a gas-tight preform is assembled or otherwise provided and then placed inside the die. The preform assembly reflects a generalized shape of the part to be formed, and may be cylindrical for forming generally cylindrical parts or planar for forming generally planar parts. In embodiments in which the method uses a hinged die, a die portion may be lifted or swung open before disposing the preform inside. The die is closed by interlocking one or more connecting portions of the die segments. In embodiments in which the method uses a hinged die, a die portion may be lowered and assembled or swung closed before interlocking the die segments. In embodiments in which the connecting portions of the die segments include bores, a pin is extended through the bores of aligned die segments to interlock them. The die with the preform assembly inside it is then placed into a vacuum furnace and heated. Inert gas is introduced under pressure into the preform assembly, superplastically expanding it and forcing it to conform to the shape of the interior chamber of the die. The gas pressure is then relieved and the assembly cooled. When the assembly has cooled, the die segments are separated. In embodiments in which the method uses a hinged die, the die is swung open. The expanded assembly is then removed from the die and trimmed to separate the portion that includes the formed part from the remaining portion.
Notwithstanding the success of that invention, it has been found that the die structures can be subjected to damaging or destructive stresses during the cooling phase of the SPF process, which shortens the die life and which also can cause distortions in the products formed with a stressed die. The invention described and claimed herein addresses and solves that problem in a unique and highly effective manner, and represents an improvement on the invention defined in the U.S. Pat. No. 5,823,034. The information presented in that patent is relevant to the present invention and its disclosure is therefore incorporated herein by reference.